1. Technical Field
The present invention relates to communication, more specifically to RFID (radio-frequency identification) systems, or similar, where decoding of bi-phase (for example FM0), Manchester or similar codes are needed. The main application field of the proposed invention are RFID-readers in RFID systems.
2. Background Art
RFID systems typically include RFID tags, and RFID readers (interrogators). Tags are mostly related to specific products or services, and are tracked by readers. Most often tags get power from the reader by radio frequency (RF) field, and respond to reader's commands by modulating the radio-waves. One problem at RFID signal decoding is large tolerance of the bit-frequencies (up to 10%), which makes it difficult to decode reliably real signals containing noise and disturbances.
U.S. Pat. No. 6,501,807 “Data recovery system for radio frequency identification interrogator” describes a system where decoding of the received data bits is carried out by counting of sequential oversampled samples of the same polarity. Such system is quite tolerant to timebase variations. General disadvantage of this system is to find precisely the beginning and ending of each data-bit and it will not allow reliable decoding of data bits at low signal-to-noise ratios being typical for real RFID systems, as power transmitted by readers is limited, and received signal is relatively weak. So, the working distance of readers is limited, or more power to be transmitted is needed, that can be impractical or not allowed by RF regulations.
US Patent Application US2005269408 “RFID joint acquisition of time sync and timebase” describes estimating the synchronization timebase not according to each data-bit individually, but according to longer time-period with multiple symbols, for example over a whole preamble (e.g., lasting for 16 data bits). Using longer time period gives smaller error in estimating of the synchronization timebase, as inaccuracies are spread over longer time interval. Also the influence of the noise is smaller over longer time interval. In this solution the best similarity (maximum correlation) of the ideal preamble and real signal is searched, by using a bank of parallel digital filters (also called correlation calculation units), designed for different timebases, and the best found timebase is further used. The problem of this solution is small accuracy and speed of finding of the correct timebase for synchronization, as the correlation is influenced by noise and disturbances in the real signal, and also as the correlation value around the maximum is relatively flat and therefore is not precise enough criterium for timebase correction for real signals. Inaccurate timebase does not allow correct decoding of the data from real signals.
Thus, there is a need for new improved method and device for synchronization of a decoder of RFID decoders.